Dragon Ball GT doesn't have a corresponding manga. The story was developed specifically for the anime series. It's not based on a pre-existing manga like the original Dragon Ball and Dragon Ball Z.
Dragon Ball GT has a manga, but it's not considered as canonical as the main Dragon Ball and Dragon Ball Z manga series. It was created based on the anime rather than the other way around.
In fanfiction, anything is possible. Regarding Harry Potter having a dragon's heart, it could be a central part of a story. For example, perhaps Harry discovers he has a dragon's heart after a strange encounter with a powerful magical creature. This newfound aspect of himself could change his relationships with his friends. Hermione might be intrigued and start researching how it affects his magic, while Ron could be a bit more cautious around him at first, worried that this new part of Harry could make him more dangerous. The concept can open up a whole new world of storytelling within the Harry Potter fanfiction universe.
Dragon Ball was a very classic Japanese manga that told the story of Sun Wukong and others fighting against evil forces. The work was once popular all over the world and still has many loyal fans. If you are looking for a Dragon Ball manga, I suggest you go to your local comic bookstore or online bookstore to buy it, or you can search for the relevant electronic version online to read.
The heart of manga could be in the unique plotlines that keep you engaged or the relatable characters you root for. It can be in the way the author tells the story and makes you feel part of it.